Catalytic transformation of aliphatic alcohols to corresponding esters in o2 under neutral conditions using visible-light irradiation

Article abstract of DOI:10.1021/ja511619c

Selective oxidation of aliphatic alcohols under mild and base-free conditions is a challenging process for organic synthesis. Herein, we report a one-pot process for the direct oxidative esterification of aliphatic alcohols that is significantly enhanced by visible-light irradiation at ambient temperatures. The new methodology uses heterogenerous photocatalysts of gold-palladium alloy nanoparticles on a phosphate-modified hydrotalcite support and molecular oxygen as a benign oxidant. The alloy photocatalysts can absorb incident light, and the light-excited metal electrons on the surface of metal nanoparticles can activate the adsorbed reactant molecules. Tuning the light intensity and wavelength of the irradiation can remarkably change the reaction activity. Shorter wavelength light (<550 nm) drives the reaction more efficiently than light of longer wavelength (e.g., 620 nm), especially at low temperatures. The phosphate-exchanged hydrotalcite support provides sufficient basicity (and buffer) for the catalytic reactions; thus, the addition of base is not required. The photocatalysts are efficient and readily recyclable. The findings reveal the first example of using "green" oxidants and light energy to drive direct oxidative esterification of aliphatic alcohols under base-free, mild conditions.

Full text of DOI:10.1021/ja511619c

Article
pubs.acs.org/JACS
Catalytic Transformation of Aliphatic Alcohols to Corresponding
Esters in O2 under Neutral Conditions Using Visible-Light Irradiation
Qi Xiao,† Zhe Liu,† Arixin Bo,† Sifani Zavahir,† Sarina Sarina,† Steven Bottle,† James D. Riches,‡,§
and Huaiyong Zhu*,†
†School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, ‡Institute for Future Environments, and
§School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia
S
* Supporting Information
ABSTRACT: Selective oxidation of aliphatic alcohols under
mild and base-free conditions is a challenging process for
organic synthesis. Herein, we report a one-pot process for the
direct oxidative esterification of aliphatic alcohols that is
significantly enhanced by visible-light irradiation at ambient
temperatures. The new methodology uses heterogenerous
photocatalysts of gold−palladium alloy nanoparticles on a
phosphate-modified hydrotalcite support and molecular oxy-
gen as a benign oxidant. The alloy photocatalysts can absorb
incident light, and the light-excited metal electrons on the
surface of metal nanoparticles can activate the adsorbed
reactant molecules. Tuning the light intensity and wavelength of the irradiation can remarkably change the reaction activity.
Shorter wavelength light (<550 nm) drives the reaction more efficiently than light of longer wavelength (e.g., 620 nm), especially
at low temperatures. The phosphate-exchanged hydrotalcite support provides sufficient basicity (and buffer) for the catalytic
reactions; thus, the addition of base is not required. The photocatalysts are efficient and readily recyclable. The findings reveal the
first example of using “green” oxidants and light energy to drive direct oxidative esterification of aliphatic alcohols under base-
free, mild conditions.
Scheme 1. Direct Oxidative Esterification of Aliphatic
Alcohol (1-octanol as example)
1. INTRODUCTION
Esterification is one of the most essential reactions in organic
synthesis.1−3 Traditionally, esters are prepared by the reaction
of activated acid derivatives with alcohols,4 a multistep process
that often produces large amounts of unwanted byproducts.
Typically, alcohols are readily available as bulk chemicals and so
represent attractive starting materials for large-scale production.
In this regard, the direct conversion of alcohols into esters
represents a significant advance toward green, economic, and
sustainable processes.5−9 One such process involves the
selective oxidation of aliphatic alcohols to the corresponding
carbonyl compounds. Among the possible alcohol oxidation
reactions, the catalytic and selective oxidation of aliphatic
alcohols with molecular oxygen is rather challenging, especially
at neutral pH and when employing only moderate reaction
conditions.10 To date, only a very few examples are known for
the direct self-esterification of aliphatic alcohols. Of these
studies, most involve homogeneous catalytic systems (Scheme
1), such as those using iodide or bromide as oxidant,11−14 or
transition-metal-complex catalysts (Pd, Ru, Rh, Ir, etc.).15−23
Furthermore, a mild base must be added to counter the acid
generated in the process.11−23 More significantly, most
homogeneous catalysis requires harsh reaction conditions,
such as high temperatures and high pressures.15−23
continues to attract significant interest. For example, Beller and
co-workers demonstrated that easily reusable Co3O4−N@C
catalysts can efficiently drive the direct oxidative esterification
of aliphatic alcohols.24 While efficient, this reaction needs to be
Therefore, the development of an environmentally benign
Received: November 18, 2014
heterogeneous catalyst for the esterification of aliphatic alcohols
© XXXX American Chemical Society
A
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conducted under 1 bar of O2 and 120 °C (Scheme 1).24 Powell
and Stahl reported a heterogeneous catalyst consisting of Pd/
charcoal in combination with bismuth(III) nitrate and tellurium
metal that efficiently esterifies aliphatic alcohols.25 This process
could be achieved under much milder conditions (1 atm of O2,
50 °C) compared with Beller’s method, but a strong base,
potassium methoxide, was used, and the mixed solid catalysts
were not easily recycled after the reaction. Overall, the use of
heterogeneous catalysts for the direct oxidative esterification of
aliphatic alcohols is rarely reported.
Herein we describe a visible-light-driven, one-pot process for
direct oxidative esterification of aliphatic alcohols that uses
molecular oxygen as oxidant and exhibits high product
selectivity under mild conditions. Gold−palladium alloy
nanoparticles (Au−Pd alloy NPs) are used as a recyclable
photocatalyst for the esterification of aliphatic alcohols, which
proceeds under irradiation without the addition of base
(Scheme 1).
The discovery of this novel catalytic process derives from our
recent development of novel Au−Pd alloy NP photocatalyzed
aryl alcohol oxidations using visible light.26 While investigating
the selective oxidation of benzyl alcohol, we observed the trace
formation of esters as side products. Addition of a base to
remove acid was seen to increase the ester yield. We envisioned
that Au−Pd alloy NPs could be useful for the direct self-
esterification of aliphatic alcohols, driven by visible-light
irradiation. This challenging goal would be of clear significance
and broad interest, especially if the addition of base could be
avoided. In this regard, an integrated photocatalyst design that
relies on the synergy of the metal NPs and support material
should in principle be particularly effective toward this goal. On
the basis of the fact that K3PO4 or K2CO3 has been used as an
additive to enhance the catalyst performance,12,13,16,18,20,24 we
theorized that the addition of base may not be required if basic
sites are present as part of the NP-supporting material. By
combining the Au−Pd alloy NPs with such supports we could
thereby provide a new heterogeneous photocatalyst for base-
free oxidation reactions.
In the present study, we have used ion exchange to introduce
phosphate anions into hydrotalcite (with a formula of
[Mg6Al2(OH)16]CO3·mH2O and abbreviated HT) to obtain a
unique acid-buffering support material (HT-PO43−) and then
loaded Au−Pd alloy NPs onto the HT-PO43− support material.
This unique structure can effectively couple the basic sites of
the support material with the photocatalytic properties of the
alloy NPs, for the direct self-esterification of aliphatic alcohols
under irradiation. Thus, the direct esterification of aliphatic
alcohols can be driven without any additive under visible-light
irradiation and benign reaction conditions. Notably, these
heterogeneous catalysts can be easily recycled and conveniently
reused, which has been regarded as an important aspect in the
development of practical and cost-effective catalytic oxidation
processes. The results reveal a potential route toward greener
commercial process for clean and efficient production of
aliphatic esters.
mol) were dissolved in 1.0 L of deionized water to form another
alkaline solution. Both of these two solutions were heated to 75 °C.
For the preparation of HT, the acidic and alkaline solutions were
added dropwise simultaneously into 400 mL of deionized water at 75
°C to obtain the precipitation. The pH value was measured to be 10.
The suspension was aged for 3 h at 85 °C under vigorous stirring.
After being cooled down to room temperature, the gel suspension was
filtered and transferred into an autoclave that was subsequently kept at
80 °C for 16 h. The hydrothermally treated gel was washed with
deionized water (350 mL) until the washings reached a pH of 7. The
resultant white precipitate was dried in an oven overnight at 80 °C and
ground to a powder.
The HT was calcined to 450 °C (heating rate 10 °C·min−1) in a
flow of 100 mL·min−1 of dry air for 8 h in preparation, yielding mixed
oxides of magnesium and aluminum, which were used for the
subsequent ion exchange process.
Phosphate-Modified HT (HT-PO43−). The calcined HT (2.0 g) was
dispersed into 50 mL of Na3PO4 aqueous solution (0.02 mmol/L), the
suspension was stirred at room temperature for 12 h, the solid was
washed and dried at 110 °C for 10 h, and the resultant solid was
3−
ground and denoted as HT-PO4
.
3−
3−
Au−Pd@HT-PO4 Catalyst. Au−Pd@HT-PO4 catalyst was
prepared by an impregnation−reduction method. A 2.0 g portion of
HT-PO43− powder was placed in a 100 mL beaker, and HAuCl4 (15.2
mL, 0.01 M) and PdCl2 (28.3 mL, 0.01 M) aqueous solutions were
added into the beaker under magnetic stirring at room temperature.
Meanwhile, lysine (16 mL, 0.53 M) aqueous solution was added, and
the suspension was kept stirring vigorously for 30 min. The pH value
of the suspension was measured to be 8−9. Then, a freshly prepared
NaBH4 (3 mL, 0.35 M) aqueous solution was added dropwise over 20
min. After standing for 24 h, the solid was separated by centrifugation
(3000 rpm), washed with deionized water (three times) and ethanol
(once), and finally dried at 60 °C in a vacuum oven for 24 h. The dried
sample can be used as the photocatalyst directly. Monometallic Au/Pd
catalysts were prepared in a similar method using HAuCl4 and PdCl2
aqueous solutions, respectively.
2.2. Characterization of Catalysts. The particle size and
morphology of the catalyst samples were characterized with a
JEOL2100 transmission electron microscope (TEM), equipped with
a Gatan Orius SC1000 CCD camera. Scanning electron microscope
(SEM) imaging, elemental mapping, and EDS were performed using a
ZEISS Sigma SEM at accelerating voltages of 20 kV. X-ray diffraction
(XRD) patterns of the samples were recorded on a Philips PANalytical
X’Pert PRO diffractometer using Cu Kα radiation (λ = 1.5418 Å) at 40
kV and 40 mA. The diffraction data were collected from 5° to 75° with
a resolution of 0.01° (2θ). Nitrogen physisorption isotherms were
measured at −196 °C on a Tristar II 3020. Prior to each measurement,
the sample was degassed at 150 °C for 16 h under high vacuum. The
specific surface area was calculated by the Brauner−Emmet−Teller
(BET) method from the data in a P/P0 range between 0.05 and 0.2.
Temperature-programmed desorption of ammonia (NH3-TPD) was
conducted on a Micromeritics AutoChem II 2920 chemisorption
analyzer to determine the acidic properties of the catalysts. Catalyst
samples were activated at 450 °C for 1 h in a vacuum. Ammonia was
adsorbed at 1 mbar and 100 °C for 1 h. For desorption, the samples
were heated to the corresponding temperature from 100 to 600 °C at a
rate of 10 K·min−1; desorbing gases were monitored with a Pfeifer
mass spectrometer. A Varian Cary 5000 spectrometer (with BaSO4 as
the reference) was used to collect the data for the diffuse reflectance
UV−visible (DR−UV−vis) spectra of the samples.
2.3. Photocatalytic Reactions. The photocatalytic reaction was
conducted in a light-reaction chamber. A 20 mL Pyrex glass tube (φ,
12 mm) was used as the reaction container. After adding the reactants
and catalyst, the tube was filled with O2 and sealed with a rubber
septum cap. Then the tube was stirred with a magnetic stirrer and
irradiated under a halogen lamp (Nelson Industries, 500 W tungsten
linear halogen lamp, with wavelength in the range 400−750 nm; the
light intensity was measured to be 0.5 W/cm2). In order to control the
reaction temperature carefully, an air conditioner was set and attached
to the light-reaction chamber. The reaction temperature under
2. EXPERIMENTAL SECTION
2.1. Preparation of Catalysts. Mg−Al HT. The Mg−Al HT with
a Mg/Al ratio of 3 was produced using a sol−gel process following
established procedures with some modification.27,28 First, Mg(NO3)2·
6H2O (115.4 g, 0.45 mol) and Al(NO3)3·9H2O (56.3 g, 0.15 mol)
were dissolved in 0.6 L of deionized water to form an acidic aqueous
solution. Then NaOH (60.0 g, 1.5 mol) and Na2CO3 (26.5 g, 0.25
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irradiation was maintained the same as the reaction in the dark to
make sure that the comparison is meaningful. All the reactions in the
dark were conducted using a water bath placed on a magnetic stirrer.
In order to strictly avoid exposure of the reaction to light, the tube was
wrapped with aluminum foil. After the reaction, 0.5 mL aliquots were
collected and then filtered through a Millipore filter (pore size 0.45
μm) to remove the solid photocatalyst particulates. The products were
analyzed by an Agilent 6890 gas chromatography (GC) with HP-5
column. An Agilent HP5973 mass spectrometer was used to identify
the product.
Action spectrum experiments were conducted with light emission
diode (LED) lamps (Tongyifang, Shenzhen, China) with wavelengths
360 5, 400 5, 470 5, 530 5, 590 5, and 620 5 nm used as
the light source. The environmental temperature was measured to be
45 2 °C, and all the other reaction conditions were kept identical
with the typical procedures. The apparent quantum efficiency (AQE)
was calculated as AQE = [(Nlight − Ndark)/(the number of incident
photons)] × 100%, where Nlight and Ndark are the number of products
formed under irradiation and in the dark, respectively.
3. RESULTS AND DISSCUSSION
Figure 1. (a) TEM image of the samples. (b) Particle size distribution
of the NPs based on the statistical analysis of TEM images (by
measuring >350 isolate particles in the images of the sample). (c) HR-
TEM image of the alloy NPs. (d) HR-TEM image of an alloy particle
indicated in part c (red square).
3.1. Catalyst Synthesis and Characterization. HT solids
are known to possess surface basic properties that can be fine-
tuned by their compositions.29−31 In this study, the phosphate-
modified HT support (HT-PO43−) was prepared by utilizing
the “memory effect” of HT. The HT solid was calcined to 450
°C, yielding mixed oxides of magnesium and aluminum. When
the mixed oxide powder was dispersed into Na3PO4 aqueous
solution, the layered double hydroxide structure was restored,
but the anions between the layers are phosphate anions. The
resultant solid was used as a support for Au−Pd alloy catalysts
(Scheme 2).
3−
Scheme 2. Preparation of Au−Pd@HT-PO4 Catalysts
Figure 2. (a) Scanning electron microscopy (SEM) image of a typical
3−
Au−Pd@HT-PO4 sample and the corresponding energy dispersive
spectrometer (EDX) mapping of Mg, Al, P, Au, and Pd elements. (b)
EDX spectrum.
As shown in the representative TEM images of the catalysts
(Figure 1), the alloy NPs are distributed evenly on the HT
surface (Figure 1a), and the average diameter of the NPs is
approximately 4−5 nm (Figure 1b). The high-resolution TEM
(HR-TEM) images reveal the atom lattices of Au−Pd alloy NPs
(Figure 1c,d). The lattice fringe spacing of 0.23 nm
corresponds to the interplanar distance of (111) planes in the
Au−Pd alloy lattice (Figure 1d).
the EDX spectrum (Figure 2b); the Mg/Al ratio (3/1) and Au/
Pd ratio (1/1) are matched with the initial experimental design.
The well-defined, layered structure characteristic of HT is
confirmed for all samples by the X-ray diffraction (XRD)
patterns (Figure 3). It is clear that all diffraction peaks could be
indexed to the HT structure, and the structure was restored
after introducing phosphate anions and remained unchanged
after the metal NPs were loaded, although the intensity of the
diffraction peaks decreased and their widths increased. No
reflections assignable to Au and Pd are present in the XRD
patterns, possibly because the low metal content was below the
detection limit and/or due to poor crystallinity of the metal
NPs on the surface of HT.
To investigate the elemental composition in the as-prepared
photocatalyst, energy-dispersive X-ray spectroscopy (EDX)
3−
elemental mappings of the Au−Pd@HT-PO4 catalyst were
performed (Figure 2a). EDX elemental mapping of the SEM
image shows that all the components are homogeneously
distributed throughout the sample. The phosphorus element is
clearly evidenced in the mapping data, and the phosphorus
signals indicate that the doped phosphate anion is uniformly
distributed in the HT sample. The percentages of Mg, Al, P,
Au, and Pd elements in the catalysts were also analyzed from
The light-absorption properties of the samples can also
confirm the formation of Au−Pd alloy NPs. As shown in Figure
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in the solar spectrum. The presence of the support and its
interaction with the NPs can substantially shift and broaden the
light-absorption peaks.32 The spectrum of the Au−Pd alloy
NPs sample is apparently different from the spectra of metal
NPs of either pure component. The dielectric constant of the
alloy NPs is different from the pure metal NPs, and the light
absorption of the metal NPs depends substantially on their
dielectric constant. In the spectrum of the alloy sample, the
characteristic localized surface plasmon resonance (LSPR)
absorption peak of Au NPs at about 520 nm is weaker
compared with the spectrum of a pure Au sample. The main
absorption peak of the pure Pd NPs on the support is at about
300 nm in the UV region, and its light absorption at solar
wavelengths occurs through the light excitation of the electron
to higher energy levels.33 Obviously, the absorption of the alloy
NPs in the visible range is more intense than that observed for
pure Pd sample. This means that the alloy NPs have a better
ability to gain light energy to enhance their catalytic
performance when irradiated with visible light.
Figure 3. XRD patterns of the photocatalysts.
4, the HT-PO43− support exhibits weak absorption in the range
of visible-light wavelengths above 400 nm, and therefore, the
3.2. Photocatalytic Performance. Exploratory photo-
catalytic experiments with the different catalytic materials were
performed using the oxidative esterification of 1-octanol to give
octyl octanoate as a model reaction. For comparison, the dark
reactions were conducted without irradiation and all the other
reaction conditions were kept identical. Typically, the reaction
temperature in the dark was maintained the same as the
reaction under irradiation by using a water bath.
To understand the effect of various bases on the performance
of the catalytic system, we used Au−Pd@ZrO2 catalysts with
various base additives [Table S1, Supporting Information (SI)]
and found that when using K3PO4 as the base the photo-
catalytic reaction exhibited the best performance, and this is in
agreement with the literature report that K3PO4 is the optimal
base additive for the oxidative esterification.20,24 The perform-
ance of the alloy NPs on the unmodified HT was also
examined. As can be seen in Table 1, Au−Pd@HT catalyst
exhibits a slightly better performance under visible-light
irradiation (entry 1) than in the dark for the direct oxidative
Figure 4. Normalized diffuse reflectance ultraviolet−visible (DR-UV/
vis) extinction spectra of the photocatalysts.
support itself cannot contribute to the photocatalytic activity. In
contrast, all the NP photocatalysts display strong absorption in
the UV and visible ranges of the spectrum, indicating that the
NPs are able to utilize most of the irradiation energy delivered
a
Table 1. Activity Test and Catalyst Screening for Oxidative Esterification of 1-Octanol
entry
1
catalyst
additive
incident light
conversion (%)
selectivity (%)
Au−Pd@HT
Au−Pd@HT
−
−
K3PO4
visible
dark
48
45
78
57
94
62
35
24
84
60
54
46
0
54
3
2
3
4
5
6
7
visible
dark
62
25
76
42
72
68
58
31
63
34
0
K3PO4
3−
Au−Pd@HT-PO4
−
−
−
−
−
−
−
−
visible
dark
3−
Au@HT-PO4
visible
dark
3−
Pd@HT-PO4
visible
dark
3−
3−
Au@HT-PO4 + Pd@HT-PO4
visible
dark
3−
HT-PO4
−
−
visible
dark
0
0
a
Reaction conditions: photocatalyst, 100 mg; reactant, 0.2 mmol; additive, 50 mg; solvent, 2 mL of α,α,α-trifluorotoluene; 1 atm of O2; environment
temperature, 55 °C; reaction time, 24 h; and light intensity, 0.5 W/cm2. The conversions and selectivity were calculated from the product formed
and the reactant converted, as measured by gas chromatography (GC).
D
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esterification of 1-octanol. Adding K3PO4 increases the reaction
Table 2. Photocatalytic Base-Free Direct Oxidative
Esterification of Various Aliphatic Alcohols
a
activity of this catalyst (entry 2). To our delight, the alloy NPs
3−
on the PO43−-modified HT, Au−Pd@HT-PO4 catalyst,
exhibit optimal performance for the one-pot oxidative
esterification. Excellent conversion (94%) and good selectivity
(76%) were achieved without any additional base additive
under visible-light irradiation (entry 3). A much lower activity
was observed for the reaction in the dark (entry 3). Several
commonly used solvents were examined for the reaction, while
other reaction conditions were maintained unchanged (Table
S2, SI), and it was found that α,α,α-trifluorotoluene exhibited
the best performance. The catalytic activities of the
monometallic Au and Pd catalyst as well as a mixture of
monometallic Au and Pd catalysts (entry 4−6) are obviously
lower than that of the alloy NP catalyst. Interestingly, Pd@HT-
3−
3−
PO4 is a superior catalyst to Au@HT-PO4 both under the
visible-light irradiation and in the dark. This could be attributed
to the better ability of Pd NPs to activate molecular
oxygen34−36 and the fact that irradiation can enhance the
catalytic performance of Pd NPs.33
a
3−
Reaction conditions: Au−Pd@HT-PO4 photocatalyst, 100 mg;
reactant, 0.2 mmol; solvent, 2 mL of α,α,α-trifluorotoluene; 1 atm of
O2; environment temperature, 55 °C; reaction time, 24 h; and light
These results indicate that the alloying of the two metals can
further enhance the catalytic activity, which is in line with our
previous reported results.26 The alloy NP surface has greater
charge heterogeneity than pure metal NP surface, which can
result in a stronger interaction between the surface of NPs and
the adsorbed reactant molecules, facilitating the reaction.
Compared with recently reported heterogeneous catalysts for
the direct oxidative esterification of 1-octanol at 120 °C under 1
bar of O2, which achieved a octyl octanoate yield of 75%,24 the
Au−Pd alloy NP photocatalyst in the present study is efficient
and much more environmentally friendly. We also conducted
the reactions with Au−Pd@HT-PO43− catalyst under simulated
sunlight irradiation: under an irradiance (light intensity) of 0.45
W/cm2 (55 °C), 82% conversion was obtained, with 80% octyl
octanoate selectivity, which is much higher than that of the
control experiment in the dark (Table S3, SI). To the best of
our knowledge, the metal NPs or Au−Pd alloy NPs on HT-
based materials have not been found to be efficient base-free
oxidation catalysts under mild conditions. The present study is
also the first successful example of the direct oxidative
esterification of aliphatic alcohols under base-free conditions
with visible light. Notably, the novel catalyst system is highly
stable and can be reused several times (see below).
b
intensity, 0.5 W/cm2. The yields were calculated from the product
formed and the reactant converted, as measured by GC; the values in
parentheses are the results in the dark.
Table 3. Photocatalytic Base-Free Selective Oxidative Cross
a
Couplings of Aryl Alcohols with Alkyl Alcohols
The direct oxidative esterification of a series of aliphatic
alcohols catalyzed by the Au−Pd@HT-PO4 catalyst was also
investigated. As shown in Table 2, good yields of the
corresponding aliphatic esters were achieved. Notably, the
product yields for the reactions under irradiation are much
higher than those of typical thermal reactions undertaken in the
dark.
a
3−
3−
Reaction conditions: Au−Pd@HT-PO4 photocatalyst, 50 mg;
reactant, 0.2 mmol; 2 mL of solvent; 1 atm of O2; environment
temperature, 55 °C; reaction time, 24 h; light intensity, 0.5 W/cm2.
b
The yields were calculated from the product formed and the reactant
converted, as measured by GC; the values in parentheses are the
results in the dark. Methanol as solvent. Ethanol as solvent. 1-
Octanol as solvent. n-Heptane as solvent.
c
d
e
f
The successful self-esterification of aliphatic alcohols also
inspired us to explore the selective oxidative cross-couplings of
aryl alcohols with alkyl alcohols to yield different alkyl esters.
Such selective cross-esterification reactions have been scarcely
studied in the past due to the potential problem of oxidizing
one alcohol in the presence of another.24 Moreover, most of the
reported selective cross-esterifications require high reaction
temperature and high oxygen pressure.18,24,37 In the present
study, the oxidative cross-esterifications of benzyl alcohol with
methanol and ethanol as well as 1-octanol were investigated,
which yield corresponding aryl esters in different solvents. By
However, when the methoxy group was situated in the ortho
position, the ester yield declined to 40% (Table 3, entry 2),
which may due to the stereohindrance effect of the substituent.
Here, the esterification with aliphatic 1-octanol is known to be
much more challengeable, yet we can still achieve a decent yield
of 30% using visible-light irradiation (Table 3, entry 4). Finally,
we also performed the self-esterification of benzyl alcohol
(Table 3, entry 5); again, the catalyst exhibited high activity and
selectivity under visible-light irradiation, achieving a 43% yield
of the corresponding benzyl benzoate.
3−
3−
using Au−Pd@HT-PO4 catalyst under visible-light irradi-
ation, yields up to 76% were achieved (Table 3, entries 1−4).
The Au−Pd@HT-PO4 photocatalysts are also effective for
the oxidation of secondary aliphatic alcohols to ketones.
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Excellent aliphatic ketone yields were achieved under base-free,
mild reaction conditions (Table 4).
Au−Pd@HT exhibit low and moderate activity, respectively.
We also found that the oxidative esterification activity of the
Au−Pd@HT-PO4 catalyst can be greatly inhibited by the
3−
addition of benzoic acid or pyridine (Scheme S1, SI). This is
likely to arise from interference with the basic/acidic sites of the
support by reaction with the benzoic acid or pyridine31 and
potentially by surface complexation of the benzoate or
pyridinium conjugate ions limiting access by the alcohol
substrate. Therefore, a moderate population of basic/acidic
sites on the supports is necessary for the catalysis, and the basic
surface sites from PO43− are superior to other basic sites for the
catalytic performance. The unique surface character of the Au−
Table 4. Photocatalytic Base-Free Oxidation of Secondary
Aliphatic Alcohols to Ketones
a
3−
Pd@HT-PO4 catalyst is essential for facilitating the base-free
direct oxidative esterification of aliphatic alcohols. The content
3−
of PO4 in the HT support also affects the catalytic activity
(Table S4, SI). According to the results of EDX analysis, when
the phosphorus content is 0.2 wt % of the catalyst, the catalyst
exhibits the best performance; further increasing the amount of
a
3−
Reaction conditions: Au−Pd@HT-PO4 photocatalyst, 50 mg;
reactant, 0.2 mmol; solvent, 2 mL of α,α,α-trifluorotoluene; 1 atm
of O2; environment temperature, 55 °C; reaction time, 24 h; and light
3−
b
intensity, 0.5 W/cm2. The yields were calculated from the product
PO4 can suppress the conversion rate and ester selectivity.
Thus, a small amount of phosphate exchanged into the HT
support can provide necessary basic/acidic sites for the
catalyzed reactions and thus avoid the need to add base for
efficient conversion.
formed and the reactant converted, as measured by GC; the values in
parentheses are the results in the dark.
A key attraction to heterogeneous catalysis is the possibility
of catalyst recycling. We carried out a series of experiments
using oxidative esterification of 1-octanol under irradiation to
3.4. Influence of the Light Intensity and Wavelength.
The relationship between the photocatalytic activity and the
incident light intensity (irradiance) was investigated. As
depicted in Figure 6, the oxidative esterification of 1-octanol
was conducted under different irradiances. When the irradiance
was raised, the reaction yields increased. The contributions of
irradiation to the conversion efficiency were calculated by
subtracting the reaction yield achieved in the dark from the
overall yield of the irradiated system when reactions were
conducted at identical reaction temperature.26 The product
yield obtained in the dark is regarded as the thermal
contribution. The relative contributions of visible-light
irradiation to the conversion efficiencies are shown in Figure 6.
The numbers with percentages in Figure 6 show the light
irradiation contribution. It can be seen that the higher the
irradiance, the greater the contribution of irradiation to the
overall reaction rate. When the irradiance is 0.3 W/cm2, the
light contribution for the reaction is only 46%, and when the
irradiance increased to 0.7 W/cm2, 74% of the product yield is
due to irradiation. A greater irradiance provides more light-
excited energetic electrons and creates a stronger electro-
magnetic field in the vicinity of the NPs (LSPR field
enhancement effect).38−40
3−
demonstrate the recyclability of Au−Pd@HT-PO4 catalyst.
3−
Briefly, after each reaction cycle, Au−Pd@HT-PO4 catalyst
was separated by centrifugation, washed thoroughly by ethanol
twice, and dried for subsequent reactions. As shown in Figure
5a, the catalyst was reused for several cycles without significant
3−
Figure 5. (a) The photocatalytic activity of the Au−Pd@HT-PO4
catalyst after being recycled five times. (b) Representative TEM image
of the Au−Pd@HT-PO4 catalyst after being recycled. The dark
3−
particles are the alloy NPs.
The light-excited metal electrons may facilitate the reaction
via two pathways: release energy to the lattice to thereby heat
the NPs (photothermal effect) or transfer to the reactant
molecules that are adsorbed on the NP, directly causing the
reaction of the molecules (excited electron transfer). If the
reaction is due to the photothermal effect, the wavelength of
the light employed in the irradiation has little impact on the
reaction rate when the irradiance is identical. A useful tool for
determining whether a reaction is induced due to the
photothermal effect is the action spectrum, which shows the
relationship between the wavelength of incident light and the
photocatalytic rate.41,42 In the present study, the photocatalyic
loss of activity. From the typical TEM image of the Au−Pd@
3−
HT-PO4 catalyst after being recycled (Figure 5b), the metal
NPs still distribute evenly on the surface of HT support, and no
obvious agglomeration was observed.
3.3. Influence of the Supports. Since the oxidative
esterification can proceed in the absence of added base, the
surface properties of the support materials of the photocatalysts
are expected to play a critical role in catalyst performance.
Specific surface areas of the photocatalysts (derived from
nitrogen sorption data) and the surface acidity of the samples
(measured by NH3 temperature-programmed desorption, NH3-
TPD) are provided in Table 5. The specific surface area is not
the decisive factor on the performance, as the optimal
photocatalyst has a relatively small specific surface area. Basic
sites on the support surface alone cannot simply be the
dominate factor enhancing the catalyst performance, since
MgO and HT have basic surface sites but Au−Pd@MgO and
activity of the oxidative esterification of 1-octanol using Au−
3−
Pd@HT-PO4
catalyst at 45
2 °C under different
wavelengths of light irradiation was studied. The obtained
reaction yields were used to calculate the apparent quantum
efficiencies (AQEs).33,43 The plot of AQE versus the respective
F
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Article
a
Table 5. Activity Test of Different Supports for Base-Free Oxidative Esterification of 1-Octanol
b
c
entry
1
catalyst
BET surface area (m2/g)
13
acid density (mmol/g) incident light conversion (%) ester selectivity (%) ester yield (%)
3−
Au−Pd@HT-PO4
0.91
1.00
1.07
0.03
0.20
visible
dark
94
62
48
45
51
33
20
3
76
42
54
3
72
26
26
2
2
3
4
5
Au−Pd@HT
59
201
3
visible
dark
Au−Pd@Al2O3
Au−Pd@MgO
Au−Pd@ZrO2
visible
dark
92
5
47
2
visible
dark
34
0
7
0
20
visible
dark
38
33
47
36
19
12
a
Reaction conditions: photocatalyst, 100 mg; reactant, 0.2 mmol; solvent, 2 mL of α,α,α-trifluorotoluene; 1 atm of O2; environment temperature; 55
°C; reaction time, 24 h; and light intensity, 0.5 W/cm2. The conversions and selectivity were calculated from the product formed and the reactant
converted, as measured by GC. Determined by adsorption−desorption of nitrogen. Determined by NH3-TPD.
b
c
Figure 7. Photocatalytic action spectrum for the oxidative
3−
Figure 6. Dependence of the catalytic activity of Au−Pd@HT-PO4
photocatalyst for the oxidative esterification of 1-octanol on the
3−
esterification of 1-octanol using Au−Pd@HT-PO4 photocatalyst.
The light-absorption spectrum (left axis) is the DR−UV/vis spectra of
intensity of irradiation. The light contribution = [(Ylight − Ydark)/Ylight
]
3−
the supported Au−Pd@HT-PO4 catalyst (black curve).
× 100%, where Ylight and Ydark are the product yields under irradiation
and controlled in the dark, respectively.
shorter than 550 nm mainly proceeds via the excited electron
transfer.
wavelengths is the action spectrum of the reaction. The action
spectrum shows that the dependence of the AQE of the
reaction catalyzed by the Au−Pd alloy catalysts on the
wavelength of irradiation does not track with the light
The photons with a shorter wavelength are able to excite
metal electrons to higher energy levels, and these electrons have
more chances to transfer to the antibonding orbitals, located
above the bonding orbitals on the energy scale, inducing
reaction.40 When the energy of the antibonding orbital is high,
the electrons excited by light with a longer wavelength do not
have sufficient energy for the injection. They will relax to low
energy levels and release energy to heat the lattice of the alloy
NPs, enhancing the reaction only by the photothermal effect.
Thus, the photothermal contribution to the AQE obtained
under the longest wavelength (620 nm) is the largest,
compared to that from the other wavelengths. Therefore, by
comparing the AQE observed at short wavelengths with that
observed at long wavelengths, we can estimate the contribution
from the excited electron transfer.33 The AQE values at 590
and 620 nm are low, and the AQE values at short wavelengths
(<550 nm) are much larger. The large difference between the
AQE values at short and long wavelengths means that most of
the chemical transformation at short wavelength is via the
excited electron transfer pathway. Moreover, the wavelength
dependence can also be well-explained by an electron-scattering
3−
absorption spectrum for the supported Au−Pd@HT-PO4
catalyst (Figure 7). The highest activity is observed at shorter
wavelengths, thus the photothermal effect is not the main
pathway for the reaction.
The total input energy absorbed by the photocatalysts under
irradiation at different wavelengths was similar for a given
reaction period, since the reaction temperature and irradiance
were maintained constant at a specific wavelength (Figure 7).
The influence from external heating has been excluded from
AQE values by deducting the product yield formed in the dark.
If a part of the input photon energy is converted into a thermal
effect, then the catalytic activity caused by the photothermal
effect is similar. The extraordinarily high enhancement in
activity with shorter-wavelength photons indicates that photo-
enhancement at 360 nm is not only due to a simple
photothermal effect.33 It is more likely that the photocatalytic
esterification of 1-octanol irradiated with light wavelength
G
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process through a high-energy adsorbate affinity state of the
reactant molecule with the metal NP surface44 and a direct
photoexcitation process involving high-energy electronic
transitions between hybridized metal and adsorbate states.45
The former suggested that different reactant adsorbates on the
metal NP surface will exhibit unique wavelength dependences
that are controlled by the strength of their interaction with the
primary or nonthermalized energetic electron distribution, and
the latter proposed that the strong hybridization and spatial
overlap of the reactant adsorbate and metal states that occur
due to chemical bond formation may allow direct photo-
excitation of metal−adsorbate bonds through dipole electronic
transitions involving hybridized metal surface atom−adsorbate
states. The impact of nonthermalized electrons on the
activation of metal−adsorbate bond results in wavelength
dependences that deviate from the light-absorption spectra of
the metal NPs. The unique wavelength-dependent photo-
excitation of metal NP catalysts may allow us to tune the
selectivity of catalytic reactions with irradiation.
We further compared the oxidative esterification of 1-octanol
3−
using Au−Pd@HT-PO4 photocatalyst at various temper-
atures under 400 and 620 nm LED irradiation, respectively
(Figure S3, SI). It can be seen that the catalytic performance of
the reaction irradiated with 400 nm wavelength is much better
than that irradiated with 620 nm wavelength and in the dark,
especially at lower reaction temperature. The dependence of
the catalytic activity on the reaction temperature for the
reaction irradiated with a long wavelength (620 nm) is very
similar to that in the dark.
The dependence of reaction activity on the light irradiance
and wavelength suggests that the light-excited electrons play a
dominate role in the observed photocatalytic activity.46 Since
the reaction rate is expected to depend on the population of
electrons with sufficient energy to induce the reaction of the
reactant molecules, one can increase the number of these light-
excited “hot” electrons by using higher irradiance or tuning the
irradiation wavelength to accelerate the catalytic reaction rate.
This knowledge may also help us to better understand the
reaction mechanism of the metal NP photocatalysis.33,38−40,43
3.5. Influence of Temperature. As an important feature of
the metal NP photocatalysis, the catalytic activity can be
increased by elevating the reaction temperatures.47,48 This
feature is also observed from the Au−Pd alloy NP photo-
catalysts.43 In the present study, we conducted the oxidative
3−
esterification of 1-octanol using Au−Pd@HT-PO4 photo-
catalyst at various temperatures both under irradiation and in
the dark.
Figure 8. (a) The catalytic activities of the oxidative esterification of 1-
octanol using Au−Pd@HT-PO4 photocatalyst at different temper-
3−
As shown in Figure 8a, raising the reaction temperature can
achieve higher product yields, for both the irradiated reactions
and the dark reactions. We calculated the contribution of the
irradiation effect as the diffence between the yields in the light
reaction and the dark reaction divided by the total yield under
irradiation. The light contribution = [(Ylight − Ydark)/Ylight] ×
100%, where Ylight and Ydark are the product yields under
irradiation and dark conditions, respectively. For example, when
at 55 °C, the difference between the light reaction and dark
reaction is 47%, accounting for 64% of the total yield. It can be
seen that the contribution decreases as the reaction temper-
ature was raised. To further understand the effect of reaction
temperature, we compare the action spectra for the photo-
catalytic reaction at two different temperatures (45 and 70 °C,
Figure 8b). Much lower AQEs were obtained at a higher
temperature as the increased number of product molecules
atures under visible-light irradiation and in the dark. The numbers with
percentages show the light contribution. (b) Comparison of action
spectra for the oxidative esterification of 1-octanol using Au−Pd@HT-
PO43− photocatalyst at two different temperatures. (c) A schematic of
the effect of increasing temperature on the gain of vibrational energy,
thus facilitating the reaction.
formed in the dark is deducted from the values when the AQE
of the irradiated reaction is calculated. The AQE value barely
varies with the wavelength of irradiation. As shown in Figure 8c,
at a higher temperature, more light-excited electrons can gain
sufficient energy for the transfer from metal NP to adsorbed
molecules (the longer wavelengths can also excite metal
electrons to high energy levels), and the probability of the
electron transfer is higher than that of the lower reaction
temperature.46 On the other hand, at higher temperatures, the
H
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Article
relative population of excited vibrational states of the adsorbed
reactant molecules increases according to the Bose−Einstein
distribution.48 This means that, on average, the reactant
molecule will require less energy to surmount the activation
barrier, and the activation of the alcohol molecule should be
much easier. At high temperatures, the contribution from the
thermal effect (phonon-driven) can be greater than that from
the light irradiation as the reactant molecules may gain most
energy from heating to overcome the activation energy barrier.
In this case, the thermal energy is sufficient enough to induce a
significant population of vibrationally excited states of the
reactant molecules,46,49 while the light-excited electrons can
also contribute to accelerating the reactions.50,51 The light-
excited electrons in the high-energy tail of a thermal Fermi−
Dirac distribution can induce reactions by a transient
population of normally unoccupied states.50 We can see that
light contribution is 72% when the reaction is limited to 35 °C
(Figure 8a). At lower temperatures, the excited electron
transfer dominates the photocatalytic activity and the thermal
effect (and photothermal effect as well) contributes much
less.51 For those reactions in which the interaction of the light-
excited electrons of the metal NPs with the adsorbed reactant
molecules can induce the reaction, high reaction temperature is
not a prerequisite for efficacy.
The above results also demonstrate that the alloy NPs have
the capacity to couple the stimuli of irradiation and heat to
drive the catalytic reaction due to the continuum of metal
electron energy levels.43,47 This property is apparently different
from those of traditional semiconductor photocatalysts, which
may show the potential of the NP photocatalysts to utilize the
infrared radiation from sunlight to facilitate chemical reactions
under ambient conditions.
3.6. Proposed Reaction Pathway. To investigate the
reaction pathway, we studied the evolution of the products
during the time course of the oxidative esterification of 1-
3−
octanol using Au−Pd@HT-PO4 photocatalyst under irradi-
ation with a short-wavelength LED (400 nm) and a long-
wavelength LED (620 nm), respectively (parts a and b of
Figure 9), and the results are compared with that of the
reaction in the dark (Figure 9c).
We found that the conversion of the reaction irradiated with
a short wavelength is higher than that irradiated with a long
wavelength and in the dark. Furthermore, the aldehyde is the
main intermediate during the reaction course, both under light
irradiation and in the dark. This suggests that the alcohol is first
oxidized to aldehyde during the process and then the aldehyde
further reacts with another alcohol molecule to achieve the
direct esterification.
A tentative mechanism for the direct oxidative esterification
of alcohols is proposed on the basis of the literature
precedent13,15,21,24 and is depicted in Scheme 3. The oxidative
esterification reaction may proceed through an oxidation of
alcohol to aldehyde (IV) and then a condensation reaction
between aldehyde and another molecule of alcohol, which
results in the formation of a hemiacetal intermediate (V),
followed by oxidative dehydrogenation to give the correspond-
ing ester. In this case, the alcohol molecule is adsorbed on the
Au−Pd alloy NP surface because the alloy NP surface has a
strong binding affinity. The selective oxidation of alcohol to
aldehyde on the alloy NP surface is likely to proceed first (I−
IV). Irradiation can facilitate the cleavage of the O−H bond for
the insertion step, which leads to the formation of metal
alkoxide and metal hydride species (II). The surface basic sites
Figure 9. Time course for the catalytic activities [reaction conversion
and product selectivity (red, ester; gray, aldehyde)] of the oxidative
3−
esterification of 1-octanol using Au−Pd@HT-PO4 photocatalyst
under irradiation with 400 nm LED (a), 620 nm LED (b), and in the
dark (c) at 55 °C.
of the support can bind the hydrogen atom of the reactant
molecule and also facilitate the O−H cleavage on the metal
surface, as per reports in the literature.52 Light-excited electrons
can promote hydrogen abstraction from the α-H of the metal
alkoxy species, which then yields the aldehyde (III, IV). DFT
calculations also suggest that the transfer of a light-excited
electron from metal NP to the reactant molecule adsorbed on
the NP surface can facilitate the cleavage of the C−H bond.26
The presence of surface basic sites also lowers the barrier for
the activation of the C−H bond of the metal alkoxide
intermediate to form the aldehyde over the metal NP surface.50
Then the formed aldehyde on the metal NP surface can react
with another molecule of alcohol and form the hemiacetal
intermediate (V).13,15,21,24 Finally, the final ester product can be
obtained by oxidative dehydrogenation of the hemiacetal
intermediate, and the metal active sites are regenerated as
oxygen reacts with hydrogen on the metal NP surface.
I
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Scheme 3. Proposed Reaction Pathway
AUTHOR INFORMATION
■
Corresponding Author
hy.zhu@qut.edu.au
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the
Australian Research Council (ARC DP110104990), Institute
of Coal Chemistry, Chinese Academy of Sciences for TPD
measurement, supported by the Foundation of State Key
Laboratory of Coal Conversion (Grant No. J14-15-605).
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* Supporting Information
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